1. Field of the Invention
The present invention relates generally to IF conversion, and more specifically to a multi-stage digital RF translator for performing IF conversion.
2. Related Art
Modern communications receivers range from simple FM-stereo radios to more complex, high-bandwidth communications receivers such as those used for satellite communications. These receivers can be implemented using entirely analog technology, or portions of the receiver can be implemented using digital solutions.
The typical analog FM receiver, for example, mixes an incoming FM signal with a variable injection frequency so that the desired channel is positioned at a desired IF (intermediate frequency). This mixing is often termed a frequency conversion or a frequency translation. In more advanced solutions, multiple frequency conversions may be performed to detect the desired frequency. In these advanced receivers, the first conversion is typically performed using a variable injection frequency, while the subsequent conversions use a fixed injection frequency.
With the advent of digital technology, there has been a trend in communications technology to implement a variety of receiver functions using digital circuits. Rapid advances in solid-state and digital-signal-processing technologies have furthered efforts to digitize communications receivers.
One conventional solution implementing digital circuits digitizes the received signal and performs a quadrature mix of the digitized signal to convert it directly to nominally 0 Hz IF. This mixing technique uses digital multipliers that are fed by high speed oscillators to provide sine and cosine injection signals. The mixing is followed by one or more stages of filtering and decimation to isolate the desired receive channel. One or more of the filtering stages may be implemented using simple multiplierless filters for ease of implementation in high speed applications.
A problem with this conventional solution is that the sine and cosine signals injected at the mixer must be generated at very high frequencies (e.g., 88 to 108 MHz for a broadcast FM receiver) to obtain proper conversion results. Generating these signals can be costly and requires a large amount of integrated circuit area.
What is needed is a digital receiver solution that allows digitization and channel isolation in the digital signal processing (DSP) domain of high-frequency receive signals without requiring high-speed DSP oscillators for mixer injection signals, while at the same time using simple high-speed multiplierless filters.